The present invention relates to wireline connections for cable heads and more particularly to a release for releasing the wireline from the cable head.
Wireline operations are carried out in oil and gas wells for conveying tools downhole in the well. A wide variety of downhole tools may be supported on a wireline including tools to perform logging, setting and retrieving operations. The tools typically comprise a combination of different tubular members threaded together to form a working unit which is manipulated from the surface via the wireline. Although tools may be conveyed downhole on a tubing string which can withstand substantially higher extraction forces than a wireline, oftentimes a wireline is preferred because it saves substantial rig time in conveying tools downhole and positioning them within the well. A release is typically provided at the cable head, which connects the tools to the wireline, to permit the wireline to be disconnected from the tools such as when the tools become stuck downhole.
The safe pull of the wireline is a pull which does not exceed one-half the breaking strength of the wireline. When a tension is placed on the wireline which is over 50% of its break point, then problems begin to occur with the electrical conductors in the wireline. Also, there is the danger of breaking the wireline.
A typical wireline release is the use of a mechanical weak point in the connection between the wireline and the cable head. Typically this is a metal member which is designed to break upon a predetermined pull on the wireline. A safety margin is also required for the mechanical weak point and typically equals 66% of the amount of predetermined pull required to break the weak point and achieve a mechanical release. The correct conventional mechanical weak point must be calculated and installed prior to running the cable head and tools into the borehole on the wireline.
Thus, there are two limitations in using the typical conventional mechanical weak point release, one is the strength of the wireline itself and the other is the strength of the mechanical weak point. Assuming the cable head is located at the bottom of the borehole, the safe pull of the wireline is the lesser of 50% of the breaking strength of the wireline or 66% of the strength of the mechanical weak point plus the weight of the wireline suspended in the well. For example, assuming a 20,000 foot well, and a wireline having a break point of 22,000 pounds and a weight of 300 pounds per foot, the safe pull for the wireline is 11,000 pounds and the weight of the wireline will be approximately 6,000 pounds. Also, assuming the mechanical weak point is set at 5,000 pounds, then the safe point of the mechanical weak point is 66% of 5,000 or approximately 3,300 pounds. Thus, the limitation on the amount of pull which can be placed on the wireline is 9,300 pounds, i.e. 6,000 pounds for the weight of the wireline and 3,300 pounds for the safe pull of the mechanical weak point. In this example, the maximum pull, i.e. safe pull, on the wireline can be only 9,300 pounds. This example is over simplified because the friction of the system was not taken into account. In particular, if the cable head and tools are in a deviated hole, there may be a pull of 9,300 pounds at the surface with only 1,000 pounds being pulled on the cable head and tools because of friction on the wireline.
Various other apparatus and methods have been provided for releasing the wireline from the cable head and tools. One prior art method of releasing the wireline includes the use of a spring set at a particular tension. Once the force on the spring is exceeded, the wireline is released. This release still requires that the amount of load required to release the wireline be predetermined prior to lowering the cable head into the well. If the spring tension is exceeded, there can be a premature release of the cable head.
Another type of prior art release relies primarily on shear pins. Since wireline has fairly low tensile capabilities with respect to tubing, the shear screw or screws used in the prior art require a fairly low shear rating. This low shear rating was necessary to prevent damage to the wireline from excessive tensile stress should the downhole tool become stuck in the wellbore. Problems are encountered with shear screws having a low failure point because they are exposed to various cyclical forces which tend to affect their ultimate shear rating. The shear screws are exposed to fluids in the well which over time can affect the inherent strength of the shear screws or pins making them susceptible to failure at stresses below their rated failure point. Unexpected release can significantly delay operations, thereby costing significant sums. An unexpected release can also result in the loss of downhole tools and in extreme cases can cause severe damage to the wellbore which requires substantial time and money to repair.
It is not unusual for the cable head and tools to become stuck as they are being retrieved from the well. For example, where the pressure in the borehole is greater than the pressure in the formation, the drilling fluids tend to cake on the interior of the casing causing the tool to become lodged as it is retrieved. Further, the longer that the cable head and tool are stuck, the more difficult the retrieval becomes. Thus, it is desirable to remove the cable head and tools as soon as possible and this may be best accomplished if a high tension may be placed on the wireline. However, where a mechanical type release is used requiring the setting of the safe pull at the surface prior to lowering into the well, the amount of pull which can be applied to the wireline is limited to the safe pull of the release mechanism. Because the mechanical release has been set at a low value to insure that the wireline can be detached from the cable head at the deepest portion of the well without exceeding the maximum safe pull on the wireline, and because the length of the wireline has been reduced since the wireline may now be at a higher elevation within the borehole, only a limited amount of the safe tensile load of the wireline may be used to dislodge the cable head and tools. Any greater pull may break the mechanical release and prematurely release the cable head and tool. Thus, it is desirable to have available the maximum amount of pull possible for retrieving the cable head and tool. Further, once the cable head becomes stuck using a conventional mechanical release, the amount of safe pull must be calculated based on the depth of the cable head in the well.
Using the conventional mechanical release, a high tension must be placed on the cable to exceed the tensile strength and break the weak point. Upon breaking the mechanical release, a large shock is imparted to the cable head because of the large tension on the wireline. For example, when the cable head is stuck, the operator will fish for the tool with the wireline left in the hole. The operator lowers a grapple which grabs the top of the cable head or the tool body. Once the tools are grabbed, the operator wants to release the wireline and remove it from the hole. This makes it a lot easier to pull the tools and pipe out of the well. Thus, the operator places a large tension on the wireline to activate the mechanical release.
Other apparatus and methods are used which do not require a mechanical break point setting. One method includes attaching at the surface a cutter tool which slides down the wireline cutting the wireline on impact at the connection of the wireline to the cable head. However, time is lost when attaching such a cutter tool since the blowout preventer has to be sealed across the wire to hold back well pressure while the tool is attached to the wireline. Another disadvantage is that the cutter tool may cut the wire prematurely if it hits a restriction on its way downhole.
Another type of prior art release includes the use of bolts which are exploded to disconnect the wireline from the cable head. Explosive bolts have the advantage of allowing the application of tension on the wireline up to the amount of safe pull permitted for the wireline. However, one disadvantage is that once the signal to detonate the explosive bolts has been sent from the surface, the detonation cannot be terminated. There are concerns that the explosive bolts will prematurely detonate accidentally releasing the cable head from the wireline. Further, many safety concerns arise in using explosive bolts. A dangerous material must be used for exploding the bolts and thus requires an explosive device to be housed within the cable head. Also, there are safety issues in storing a cable head having an explosive device. Such a release system requires that many safety devices be used to ensure that adequate safety is provided.
In yet still another prior art release, a spring loaded piston is used which can be activated by pressuring up the wellbore and applying the pressure to the piston. However, the release can be prematurely activated by encountering a higher pressure downhole. In particular, the deeper the cable head is lowered into the well, the higher the borehole pressure.
In another prior art apparatus, the cable head includes a plurality of full diameter sections with one of the sections being released. However, a fall diameter tubular member is more difficult to retrieve from the well.
Major problems occur if the cable head and tools get stuck in the well and the wireline breaks upon pulling on the wireline with too much tension. Breaking the wireline and dropping the wireline in the well greatly complicates the fishing operation to retrieve the tools.
The present invention overcomes the deficiencies of the prior art.